[llvm] [RISCV] Move strength reduction of mul X, 3/5/9*2^N to combine (PR #89966)

[Philip Reames via llvm-commits]

preames wrote:

> Another regression (extracted from qemu):
> 
> ```
> ; bin/llc -mtriple=riscv64 test.ll -mattr=+m,+zba -o -
> define ptr @test(ptr %0, i64 %1) {
> entry:
>   %2 = lshr exact i64 %1, 2
>   %3 = and i64 %2, 4294967295
>   %4 = getelementptr inbounds i8, ptr %0, i64 600
>   %5 = getelementptr [80 x i8], ptr %4, i64 %3
>   ret ptr %5
> }
> ```
> 
> Before:
> 
> ```
> test:
>         srli    a1, a1, 2
>         slli.uw a1, a1, 4
>         sh2add  a1, a1, a1
>         add     a0, a0, a1
>         addi    a0, a0, 600
>         ret
> ```
> 
> After:
> 
> ```
> test:
>         slli    a1, a1, 2
>         srli    a1, a1, 4
>         slli.uw a1, a1, 4
>         sh2add  a1, a1, a1
>         add     a0, a0, a1
>         addi    a0, a0, 600
>         ret
> ```
> 
> We may fix this by handling `ISD::SHL_ADD` in `SimplifyDemandedBits` after #88791 lands:)

See https://github.com/llvm/llvm-project/pull/91626 for this one.

https://github.com/llvm/llvm-project/pull/89966